A mildly interesting tyrannosaur dentary from Ojo Alamo.

Skye McDavid, July 30, 2022

Badly damaged tyrannosaurid right dentary, ACM 7975, in the Beneski Museum of Natural History, Amherst, Massachusetts. Photo by Skye McDavid

Near the entrance to the Beneski Museum of Natural History in Amherst, Massachusetts, displayed behind glass, is ACM 7975, a badly damaged right dentary from a tyrannosaur. It's mislabeled as Gorgosaurus libratus, but it's clearly not that. It doesn't quite match Gorgosaurus, and it comes from the Ojo Alamo Formation, separated from the Dinosaur Park Formation (where true Gorgosaurus libratus have been found) by almost two thousand kilometers and ten million years.

Comparison of Tyrannosaurid dentaries, not to scale: Above, ACM 7975, an indeterminate likely Tyrannosaurine from the Ojo Alamo Formation, below, FPDM V8062, a Gorgosaurus libratus from the Dinosaur Park Formation. ACM 7975 photo is by Skye McDavid. FPDM V8062 photo is modified form Dalman & Lucas, 2017.

It's definitely a tyrannosaurid, and more likely a tyrannosaurine than an albertosaurine. Many tyrannosaurid remains, mostly isolated teeth, have been reported from the Ojo Alamo Formation, but none of these are diagnostic. A 2013 paper by Dalman, however, mentions new taxa of tyrannosaurids from the Ojo Alamo Formation. "Alamotyrannus brinkmani" is mentioned by name and a supposedly-in-press paper by himself and Lucas is cited. Among dinosaur nomina nuda, "Alamotyannus brinkmani" has become fairly famous, but little is publicly known about it. According to Mickey Mortimer's Theropod Database, the publication was postponed and when it is finally named at some point in the future, the dinosaur will receive a different name. It is unclear what the intended type material for this new genus is. (As a side note, the previously published specimens referred to "Alamotyrannus" by Mortimer were considered indeterminate by previous publications. Mortimer is a self-described lumper and I tend to err on the splitter side, so I think it would be more prudent to regard these specimens as indeterminate, though it's certainly plausible that these come from "Alamotyrannus".)

Dalman's 2013 paper which gave us a glimpse at "Alamotyrannus" also mentions ACM 7975 as a new taxon. It's not clear from the paper whether Dalman means "Alamotyrannus" or a separate new taxon. Coexistence of two tyrannosaurids is possible: Gorgosaurus and Daspletosaurus coexisted in Dinosaur Park.

ACM 7975 may or may not represent a new genus, but the problem is that it's fragmentary, poorly preserved, and it's a dentary. Dentaries are often not diagnostic to the species level in tyrannosaurs. I showed a photo of the specimen to tyrannosaur researcher Dr Dave Hone, who said the "really busted up lump" was too fragmentary and poorly preserved to name or reliably assign to any species, though it may or may not turn out to be new. I agree with this assessment. "Alamotyrannus" is the most likely candidate for what this dentary comes from, but I think it's safest to consider it an indeterminate Tyrannosaurid.


Dalman SG (2013) New Examples of Tyrannosaurus rex from the Lance Formation of Wyoming, United States. Bulletin of the Peabody Museum of Natural History 54(2):241-254

Dalman SG & Lucas SG (2017) On the dentary in the Fukui Prefectural Dinosaur Museum of Gorgosaurus libratus (Theropoda: Tyrannosauridae) from the Dinosaur Park Formation (Upper Cretaceous) of Alberta, Canada. Memoirs of the Fukui Prefectural Dinosaur Museum 16:17-27

Jasinski SE, Sullivan RM, & Lucas SG (2011) Taxonomic composition of the Alamo Wash Local Fauna from the Upper Cretaceous Ojo Alamo Formation (Naashoibito Member), San Juan Basin, New Mexico. Bulletin of the New Mexico Museum of Natural History and Science 53:216-265

Mortimer M (last updated 7/5/2022; retrieved 7/30/2022) "Tyrannosauroidea" The Theropod Database https://www.theropoddatabase.com/Tyrannosauroidea.html [archived]

Thanks to Dr Dave Hone for helpful information.